Mine was 2 hours. Once monthly we have a trainer come in, and she does several groups (depending on horse level). The greenie group was about 1.5 hours and had 2 horses, where the meter jumper group had 4 and lasted 2 hours.
I think if its a "weekly" lesson with the same trainer and same group, I'd estimate about an hour for 3-4 horses max. Warmup jumps can be done together (youshouldnt have to stand and wait for one person to jump...just space yourself out), and get on early to warm the horses up so your flat schooling isnt just warmup stuff.
In our lessons, we all do flat and warmup jumps at the same time. Then, do some small grids or exercises together. The last 40 minutes or so, we each do a course twice. I find this is plenty enough work for the horses AND riders
I meant to "appreciate" your post. Not thumbs down it. Sorry.